Add NPM install documentation to top-level README

[ChrisC]

It would be a lot easier for users to install this software if we directed them to the npm package and also provided install instructions both on the NPM package page and on the top-level README in this repo.

Additionally, the basic install instructions may require some additional OS configuration to get the AT-driver-server running. For example, suggestions for the MACOS server install instructions could be:

• run npm -g install @bocoup/macos-at-driver-server
• run at-driver install
• Go into VoiceOver Utility and uncheck Web -> General -> "Automatically speak the webpage"
• Allow JavaScript from Apple Events' in the Developer section of Safari Settings